1. Field of the Invention
This invention relates to ink jet recorders and particularly to a magnetic jet printer.
2. Description of the Prior Art
In magnetic ink jet recorders of the type shown in U.S. Pat. No. 3,959,757, to D. F. Jensen on May 25, 1976, and U.S. Pat. No. 3,928,855, issued to E. F. Helinski, H. C. Lee and J. L. Zable on Dec. 23, 1975, a continuous stream of ferrofluid ink drops is projected successively past electromagnetic selector and deflector devices. The selector, which is energized in timed relation with the flight of the ink drops, applies a magnetic field force to certain drops not used for printing causing them to be horizontally deflected to follow a new trajectory leading to a drop catcher located in advance of the print medium. All the ink drops, both selected and non-selected, i.e. unused and print drops, are then subjected to a time dependent magnetic field force gradient, as shown in U.S. Pat. No. 3,864,692, issued to J. A. McDonnell, R. E. McGuire and R. Radlinsky on Feb. 4, 1975, which deflects a grouping of drops various amounts in the vertical direction. The selected drops go to the drop catcher and the print drops become deposited on the print medium in accordance with the desired data pattern.
Heretofore, the electromagnetic deflector comprised a C-shaped magnetic core terminating in a pair of oppositely disposed pole pieces. The faces of the pole pieces are tapered to form an upwardly-extending wedge-shaped air gap, which produces a non-uniform magnetic gradient. The deflector magnetic core is located relative to the stream of drops such that the trajectories of both the selected and print drops pass through the air gap. The trajectory of the print drops is generally in the center of the air gap while the trajectory of the selected print drops is displaced to one side of the center. The magnetic core has a thickness equivalent to several drop wavelengths, thereby providing an elongate axial air gap such that a plurality of drops is always within the air gap for a time interval during which a raster scan signal such as a linear sawtooth or a staircase ramp is applied to the energizing winding on the core in accordance with the technique described in the previously mentioned McDonnell et al patent.
When the raster scan signal is applied to the energizing winding of the magnetic deflector, ink drops within the air gap become polarized and are deflected within the gap in the direction of increasing flux density, i.e. toward the narrower region of the air gap.
To print at higher print rates, the flight velocity of the drops must be increased and the spacing of the drops must be decreased. The amount of deflection, however, must be very substantially increased. The deflection force can be increased simply by aiming the drop stream to be centered closer to the narrow portion of the air gap. Doing that, however, will cause some of the ink drops to crash into the pole faces, thereby contaminating them and affecting their proper operation and the ultimate print quality. An alternative solution to get an increased deflection force is to aim the stream so that the ink drops are centered outside the air gap proximate the narrowest region of the gap. The external stream, however, presents a problem in that the unused, i.e. the selected, ink drops which were horizontally deflected by the selector, now move through an off-center part of the field in which they experience a centering force which tends to cancel the selector angle, thereby causing the unused drops to miss the drop catcher and become deposited on the print medium to undesirably affect print quality.